1. Field of the Invention
The present invention relates to a fuel cell system including a solid-oxide fuel cell (SOFC).
2. Description of the Related Art
Conventionally, as such a fuel cell system, a fuel cell system explained below is known. A solid-oxide fuel cell is formed in a bottomless or bottomed cylindrical shape, a fuel gas containing hydrogen is let through the inner side or the outer side of the fuel cell and an oxidant gas (the air) is let through the other side to cause a power generation reaction. The fuel gas is obtained by reforming a fuel gas containing hydrocarbon such as a utility gas. The reforming is performed by a reformer. The reforming is so-called steam reforming. As a fuel cell system that performs the steam reforming in the reformer, for example, Japanese Patent Application Laid-Open No. 2008-53209 discloses an example of the fuel cell system.
Japanese Patent Application Laid-Open No. 2008-53209 discloses a technique explained below. A pump is provided upstream of the reformer to supply water (steam) to the reformer. The reformer causes steam reforming (hereinafter also referred to as SR) using the water (the steam) and a fuel gas containing hydrocarbon to obtain a reformed fuel gas.
An SOFC has high power generation efficiency and use only a small amount of a fuel gas. Therefore, there is an advantage that only extremely small amounts of gas and steam have to be supplied to a reformer. For example, in the SR explained above, a required amount of water is about 8 ml per minute.
Attention is paid to a starting method peculiar to a fuel cell system including the SOFC. Since the SR is an endothermic reaction, if the SR is immediately performed in the beginning of start, the temperature of a module including the SOFC does not rise and does not rise to stable operation temperature. Therefore, in the beginning of start, only the air and gas are fed into the reformer to cause the reformer to perform partial oxidation reforming (hereinafter also referred to as POX) as a heat generating reaction.
When the POX and the SR are compared, since hydrogen generation efficiency is high in the SR, it is demanded to gradually shift to the SR according to a temperature rise in the SOFC. Therefore, when attention is paid to an amount of water supplied to the reformer, it is necessary to smoothly shift from a state in which no water is used to water supply of about 8 ml per minute. During such shift, in some case, auto thermal reforming (hereinafter also referred to as ATR) including both the POX and the SR is advanced.
In view of the circumstances explained above, it is desirable to gradually increase an amount of water supplied to the reformer from as small an amount as possible. However, actually, it is extremely difficult to perform such water supply. The fuel cell system including the SOFC has high efficiency as explained above and reaches extremely high temperature (about 700° C.). Therefore, when the fuel cell system is restarted after once started and stopped, the temperature of a water supply pipe for supplying water to the reformer rises. It is highly likely that water in the water supply pipe has evaporated. It is extremely difficult to accurately supply a small amount of water to the water supply pipe in which there is no water.
In order to accurately supply a small amount of water, a high sensitivity sensor for detecting such a small amount of water is necessary. However, if the high sensitivity sensor is used to detect a small amount of water, a flow of the air before a flow of the water is also detected. Therefore, accurate detection of an amount of water cannot be performed. Further, it is extremely difficult to adopt the high sensitivity sensor in terms of cost and the like.
On the other hand, if a general and practical low sensitivity sensor is used, such a flow of the air is not detected. However, when a small amount of water near a lower measurement limit flows into the water supply pipe, it is extremely difficult to surely detect the water. This is because a detecting mechanism (e.g., a gear or an impeller) of the sensor stands still in a state in which there is no flow of water or there is no water. Since a coefficient of static friction works well in the detecting mechanism in the standstill state, even if a very small amount of water flows, it is highly likely that the detecting mechanism does not operate because the flow does not overcome friction such as abrasion. Therefore, in particular, in an initial period of driving from a cold stop state, detection is difficult.
As a result of such an examination, the inventors found that, even with a sensor that can steadily detect a flow of a predetermined amount of water, it is difficult to surely detect the flow of the predetermined amount of water in an initial operation state. If such inability to surely detect an amount of water supplied to the reformer is left unattended, it is likely that insufficiency of water or excessive supply of water occurs. When water is insufficient, in some case, carbon deposition occurs in the reformer and a fuel cell and a catalyst are broken. When water is excessive, in some case, the temperature of a module including the fuel cell does not rise and stable operation cannot be performed.